Detoxification of effluents in a macrophyte treatment system

Piggery effluent was phytotoxic in an aquatic plant filter system in which Typha domingensis Pers. was grown hydroponically on wastewater percolating through a gravel substrate. A mixture of acetic, propionic and butyric acids at concentrations equivalent to those found in the effluent (16.1, 6.7 and 3.3 mM, respectively) reduced the transpiration, fresh weight production and root fibrosity of Typha seedlings in nutrient solution, the effect being more severe at high salinity. Bioassay with germinating wheat suggested that acetic acid was the major phytotoxin, with propionic acid also contributing. The capacity of the system to stabilise organic acids was investigated in laboratory incubation experiments. Algae occurring in the aufwuchs on the gravel substrate played a critical role in dissipation by supplying oxygen to associated bacteria, rather than by heterotrophy, since decay was inhibited by addition of the photosynthetic inhibitor, terbutryne. Implications for the design and operation of efficient macrophyte filter systems are discussed.